1. Field of the Invention
This invention relates to ferritic heat-resisting cast steel, and more particularly, to heat-resisting cast steel which is suitable for use in making an exhaust manifold for an automobile engine, a turbine housing, or the like.
2. Description of the Prior Art
It was usual to employ high-Si nodular graphite cast iron, Niresist (or Ni-Resist), etc. for making an exhaust manifold or a turbine housing. The development of an automobile engine having a higher output and a lower fuel consumption has, however, given rise to a demand for materials having a higher level of heat resistance. High-Ni and high-Cr austenitic heat-resisting steels have been well known for their high heat-resistance, but have been too low in castability and machinability to be acceptable for the efficient manufacture of engine parts at a reasonable cost.
High-Cr ferritic heat-resisting cast steels have come to draw attention for their reasonably high castability and machinability. These steels, have, however, been found still unsatisfactory in heat resistance, since they show a sharp reduction in strength at temperatures over the range of 550.degree. C. to 650.degree. C. (see, for example, "Handbook of Stainless Steels", Nikkan Kogyo Shinbunsha, pp. 513-521, and "Gakujutsu Geppo" (Monthly Report on Sciences), Vol. 43, No. 1, pp. 18-22).
Improved ferritic heat-resisting cast steels have, therefore, been proposed. For example, Japanese Patent Laid-Open No. 159354/1989 has proposed ferritic heat-resisting cast steel containing basically 0.06 to 0.20% C, 0.3 to 1.0% Mn, 0.4 to 2.0% Si and 15 to 22% Cr, and further about 0.01 to 1.0% of another element providing improved heat resistance, such as Nb, V, Ni, Mo or W, all by weight. This steel has, however, a number of drawbacks. It contains W at the sacrifice of the oxidation resistance which is one of the great advantages of the ferritic heat-resisting cast steels in general. The relatively high proportion of manganese which it contains is likely to add to its hardness and thereby lower its machinability. The relatively high proportion of nickel which it may contain is likely to cause it to have a lower eutectic transformation temperature and thereby lack structural stability.